The background of the invention will be discussed in two parts.
1. Field of the Invention
This invention relates to the measurement of the vortex shedding frequency of vortex flow meters, and more particularly to a differential pressure sensor for determining the vortex shedding frequency.
2. Description of the Prior Art
Vortex meters of current design use the localized fluid pressure fluctuations associated with the vortices produced by the bluff body to detect the vortex shedding frequency. The methods used to sense these pressures vary, but most use either a sealed, fluid-filled diaphragm-type differential pressure sensor, or use strain gages or force sensors to detect the forces caused by these pressures in some element of the bluff body.
A fluid-filled diaphragm type sensor is shown and described in U.S. Pat. No. 3,948,098, issued to Richardson, et al., on Apr. 6, 1976, such patent being entitled "Vortex Flow Meter Transmitter including Piezo-electric Sensor".
Fluid filling and sealing of any sensor is a critical and difficult operation that can be costly. Also, the fill fluid limits the temperature range of the sensor, due to the thermal expansion, viscosity and chemical breakdown (or polymerization) characteristics of the fluid. The stresses in the bluff body tend to be rather low, and while it is possible to design higher stresses into some element of the bluff body, or bodies, the most highly stressed components are exposed to the process media, and their materials must be chosen for corrosion properties and not for strength and fatigue properties.
Furthermore, it is difficult to make the same sensing scheme work for all sizes of meters. Diaphragm sensors cannot be made small enough to fit in the bluff bodies of the smallest meters, so the sensor must be relocated or the geometry of the bluff body must be changed. The high stress bluff body elements in some designs become too small to be fitted with strain gages, or the forces or motions generated become too small in the small meters, or too large in the large meters for the force sensor used.
U.S. Pat. No. 3,796,096, entitled "Vortex Flowmeter," was issued to Sielaff et al on Mar. 12, 1974, and discloses a flow meter having a plate like member pivotally suspended about a central axis within a conduit, with a light beam photoelectric detection arrangement for measuring the frequency of oscillation of the plate like member.
U.S. Pat. Nos. 3,867,839; 3,888,120; 3,946,608; and 4,033,189 are all commonly owned and disclose vortex-type flowmeters utilizing a conduit forming a passage for the fluid to be measured, with an obstacle assembly mounted therein, the obstacle assembly including a front section, and a rear section. The front section is configured in the form of a triangularly cross-sectioned block extending across the conduit and fixed to opposites sides thereof, with the apex of the block facing the incoming fluid. The rear section is shorter in width and is cantilevered from the front section by an interconnecting flexible web portion attached to the rear of the block. The rear section is provided with a rear tail extending in the direction of fluid flow. A gap is formed intermediate the front and rear sections, with the gap tending to trap the vortices, while strengthening and stabilizing the vortex street produced thereby. Oscillations in the rear section are sensed by different methods.
Other patents which disclose vortex flowmeters, and are exemplary of the prior art are: U.S. Pat. No. 3,992,939, entitled "Apparatus for Producing a Mass Flow Rate Signal With or Without a Density Signal," issued to November on Nov. 23, 1976; U.S. Pat. No. 4,201,084 entitled "Vortex Flow Meter," issued to Ito et al on May 6, 1980; U.S. Pat. No. 4,339,957, entitled "Vortex-Shedding Flowmeter with Unitary Shedder/Sensor," issued to Herzl on July 20, 1982; and U.S. Pat. No. 4,380,935, entitled "External Sensing Vortex Flowmeter," issued to Sgourakes et al on Apr. 26, 1983.
In such prior art meters, strain gauges or piezoelectric sensors are coupled to flexing or vibrational elements for providing inputs to electrical circuitry which inputs are converted to a flow rate. In most of such prior art meters, the sensing element, usually a piezoelectric ceramic element, is exposed to the full temperature of the process fluid. Furthermore, many of the designs are as sensitive to accelerations from pipeline vibrations as they are to vortex pressures. In addition, many of the prior art meters require complete disassembly of the meter if the sensor should fail.
In accordance with an aspect of the present invention, it is an object to provide a new and improved cantilever beam, insertable vortex meter sensor.